#pragma GCC optimize(2)
#pragma GCC optimiez(3)
#include<bits/stdc++.h>
#define int long long
#define fi first
#define se second 
#define IOS ios::sync_with_stdio(false),cin.tie(0),cout.tie(0)
using namespace std;

const int N=1010;
int a[N],b[N],n;
double ret;
void S(){
	for(int i=1;i<=n;++i){
		int j=(i+1);
		if(j>n)j=1;
		ret+=a[i]*b[j];
		ret-=a[j]*b[i];
	}
	ret=fabs(ret/2);
}
signed main(){
	IOS;
	cin>>n;
	for(int i=1;i<=n;++i){
		cin>>a[i]>>b[i];
	}
	S();
	printf("%.3lf\n",ret);
	return 0;
}
